1. Field of the Invention
The present invention relates to an apparatus for removing the load effect, and more particularly, to an apparatus for removing the load effect through addition or subtraction of the number of sustain pulses.
2. Description of the Background Art
FIG. 1 is a view for explaining a method of representing the gray scale in a plasma display panel according to the prior art.
As shown in FIG. 1, the plasma display panel is driven with one frame being divided into several sub-fields SF1 to SF8 having a different number of emission in order to implement the gray scale of an image. Each of the sub-fields is divided into an address period where a discharge cell is selected, a sustain period where the gray scale is represented according to the number of sustain pulses, and the like.
The whole screen of the plasma display panel is composed of several cells. The ratio of cells, which are selected to emit light, among the cells is called a load. The higher the number of cells that are selected to emit light, the higher the load. Brightness is controlled by adjusting the number of sustain pulses. Although the number of sustain pulses is the same, brightness varies depending on a load because the amount of externally applied power is constant.
FIG. 2 is a graph illustrating variation in a brightness depending on a load in a given number of sustain pulses.
If the number of cells to be discharged increases and a load increases accordingly, more power is needed. Thus, there occurs a phenomenon that, assuming that the number of sustain pulses is the same, the higher the load, the lower the brightness. This is called the load effect.
Actually, if there is a desired brightness level in a given sub-field when a plasma display panel operates, the number of sustain pulses, which corresponds to the brightness level, is defined and then used to represent a brightness.
However, the brightness represented based on the number of sustain pulses, which is currently defined, varies according to the load, as shown in the graph of FIG. 2. This results in distortion of the gray scale. That is, if the number of sustain pulses is constant, brightness must be constant even when the load varies. However, since the brightness is changed according to the load, the picture quality is lowered. In the prior art, in order to prevent this load effect phenomenon, lowering in brightness depending on an increased load is compensated by increasing the number of sustain pulses as the load increases on the basis of a minimum value of the load.
However, if lowering in brightness depending on a load is compensated for through the conventional method, the total number of sustain pulses increases since a reference brightness level is based on a brightness when the load is the lowest. This results in increased power consumption. Furthermore, the brightness of the whole screen becomes brighter because of the increased number of the sustain pulses. Accordingly, there is a problem in that a flickering phenomenon of a screen becomes more severe.